1. Field of the Invention
The invention relates to the side entry, rod type of specimen holder commonly used in high resolution electron microscopes and more particularly to the drift in the image of the specimen that occurs when, for various reasons, the temperature of such a holder is different from that of the microscope.
2. Description of Prior Art
Modern electron microscopes are capable of resolving the spacing between atoms providing the drift rate of the specimen is less than about 0.1 nm. A widely used type of electron microscope specimen holder consists of a rod which is inserted through a side entry airlock and placed against a specimen translation stage situated in the center of the microscope objective lens. The device is often described as a rod type, side entry specimen holder. Typically, when the specimen rod is installed after specimen loading its temperature is different from that of the microscope and there is a long period of time during which thermal expansion or contraction cause movements of the specimen which exceed the microscope resolution. These movements appear as drift in the image of the specimen and prevent the full resolution capabilities of the microscope from being realized. Even greater amounts of specimen drift can occur in special forms of side entry holders used for heating and cooling specimens in the electron microscope. In these holders the drift is particularly toublesome because it remains relatively high even after long periods of observation.
A device for reducing thermal drift in the electron microscope was described in an article entitled "Side Entry Specimen Stages" by P. R. Swann in Kristal und Tecknik 14, 10, 1979 pp. 1235-1243, herein incorporated by reference. It consists of an electronically controlled piezoelectric crystal coupled to the normal specimen translation stage of the electron microscope to produce a synthetic stage drift which exactly compensates the natural thermally induced specimen drift. This device, although elegant in concept, is complicated to operate.
A second and simpler device is to be found in the side entry, electron microscope specimen holders manufactured by Gatan Incorporated. It consists of a vitreous silica rod embedded in the nose of the specimen holder and spanning most of the distance between the center of the specimen and the specimen translation stage. Since vitreous silica has a very small coefficient of thermal expansion the specimen drift caused by temperature changes in the specimen holder is considerably reduced. Unfortunately, the vitreous silica cannot be used to span the entire distance affected by thermal expansion because being a brittle insulator it is not a suitable material to support the specimen itself. There is, therefore, a residual amount of specimen drift which cannot be eliminated by this device. Furthermore, the silica rod used in the Gatan Incorporated specimen holders is glued into the specimen holder and since it is not freely suspended some of its effectiveness in reducing expansion or contraction is lost.